Chain transmissions in which a chain is in meshing engagement with a driving sprocket and one or more driven sprockets are used widely. Regardless of the environment in which they are used, it is generally desirable to reduce the noise level generated in the operation of such chain transmissions
When the chain transmission is used as a timing transmission in an internal combustion engine, to transmit power from an engine crankshaft to one or more camshafts for operating intake and exhaust valves, it needs to meet the demand for high power output and high combustion efficiency, and meet environmental concerns as well. Thus, whereas the load on the timing transmission has increased, there is a conflicting demand for reduction of the noise generated by the transmission to a negligible level.
Countermeasures for reducing noise in an automobile engine have included the use of sound-absorbing materials such as rubber. However, where the load on the timing transmission is at a high level, and chain tension is high, it has become difficult to suppress noise sufficiently.
These chain transmissions are defined in Japanese Industrial Standards (JIS) as JIS B1801-1997 (Transmitting roller chain and Bushing chain), and the sprocket tooth forms (S tooth form and U tooth form) are also defined in a reference (“Shapes and Sizes of Sprocket”) attached to JIS B1801-1997. Further, tooth forms for chains and sprockets according to the International Standards Organization (ISO tooth forms) are defined in ISO 606:1994 (E). Conventional chain transmissions utilizing roller chains or bushing chains together with sprockets are generally made in accordance with these standards.
FIGS. 5 and 6 depict a conventional chain transmission, composed of a standard roller chain 150, and a standard sprocket 500 having an ISO tooth form. FIG. 6 is an enlarged view of a portion Y in FIG. 5.
The ISO tooth forms shown in FIGS. 5 and 6 are defined by the following expressions from ISO 606:1994 (E).d=p/sin(180°/z)(Pitch circle diameter)df=d−d1 (Diameter of tooth gap bottom circle, or “root diameter”)dc=df Caliper diameter for a sprocket having an even number of teethdc=d×cos(90°/z)−d1 Caliper diameter for a sprocket having an odd number of teethre(max)=0.12×d1(z+2) Maximum of radius of tooth head arcri(min)=0.505×d1 Minimum of radius of tooth gap bottom arcre(min)=0.008×d1(z2+180) Minimum of radius of tooth head arcri(max)=0.505×d1+0.069(d1)1/3 Maximum of radius of tooth gap bottom arcIn the above formulae,    p is the roller chain pitch,    d is a pitch circle diameter,    d1 is the outer diameter of a chain roller,    df is the diameter of the tooth gap bottom circle, i.e., the root diameter,    dc is a caliper diameter,    re(max) is the maximum of the radius of the tooth head arc,    ri(min) is the minimum of the radius of tooth gap bottom arc,    re(min) is the minimum of the radius of the tooth head arc,    ri(max) is the maximum of the radius of the tooth gap bottom arc, and    z is the number of sprocket teeth.
In FIGS. 5 and 6, pa is the chordal pitch of the sprocket, which is equal to the chain pitch p. As apparent from the above expressions, in the standard sprocket 500, shown in FIG. 6, the tooth gap bottom 503 of the ISO tooth form is in the form of an arc having a radius ri slightly larger than the radius (d1/2) of roller 152. The tooth surface 502 is in the formed of an arc having a radius re. The tooth surfaces 502 on both sides of a tooth gap are continuous with the tooth gap bottom 503. The diameter df of the tooth gap bottom circle, is equal to the difference between the pitch circle diameter d and the roller diameter d1. The diameter df of the tooth gap bottom circle is substantially the same as the difference between the pitch circle diameter d and two times the radius ri of the tooth gap bottom arc.
The standard chain 150 includes inner links and outer links arranged in alternating, overlapping relationship along the length of the chain. In each inner link, both ends of each of two bushings are respectively press-fit into bushing holes in a pair of inner plates. Rollers, each having an outer diameter d1, are rotatable on the bushings. In each outer link, both ends of two connecting pins are respectively press-fitted into pin holes of a pair of outer plates. The chain is assembled in such a way that each of the two connecting pins of an outer link extends through a bushing of a different adjacent inner link, so that the links are connected together. The bushings are rotatable on the connecting pins to allow articulation of the chain. The standard roller chain 150 has a uniform chain pitch p, i.e., a uniform distance between the centers of its respective rollers.
In the standard sprocket 500, as shown in FIGS. 5 and 6, the tooth gap bottom 503 and the tooth surface 502, which are continuous with the tooth gap bottom 503, are symmetrical with respect to a center line X extending radially from the rotational center O of the sprocket through center of the tooth gap bottom 503. The tooth form pitch angle θ is the angle formed by adjacent center lines X, and, is determined by the number of teeth on the sprocket. That is, the tooth pitch angle θ=360°/z. The tooth form pitch pa is the distance between intersection points a, where the radial center lines X intersect the pitch circle pc. Therefore, the tooth form pitch pa is the length of a chord corresponding to the tooth form pitch angle θ. Since, in the standard sprocket 500, the tooth form pitch angles θ are all the same, the chordal tooth form pitch pa is uniform along the circumference of the pitch circle pc. Furthermore, the chordal tooth form pitch pa is made equal to the chain pitch p.
In another approach to reduction of engagement noise, described in Japanese Examined Patent Application No. Hei 7-18478, the outer diameter the rollers of a roller chain is made larger than the standard size so that, as each roller abuts the opposed surfaces of a pair of adjacent sprocket teeth, a clearance exists between the roller and the tooth gap bottom. The tooth gap bottom is in the form of an arc having a diameter slightly smaller than the outer diameter of the roller. As the roller slides on the tooth surfaces and seats on the tooth gap bottom elastic deformation of the roller and/or the tooth surfaces takes place.
In a conventional transmission device comprising a standard roller chain and a standard sprocket, when the standard sprocket rotates in the clockwise direction as shown in FIG. 5, a following roller 152 moves, relative to a seated preceding roller, circumferentially about the center c of the preceding roller, in an arc having a radius equal to the chain pitch p. The following roller then comes into a substantially right angle collision with a tooth gap bottom. Thus, upon engagement of the following roller 152 with the tooth gap bottom the kinetic energy of the roller is transmitted to the tooth gap bottom without buffering, generating a large amount of vibration and noise.
Since the chordal tooth form pitch pa of the standard sprocket 500 is the same as a pitch p of the standard roller chain 150, each roller 152 abuts the tooth gap bottom of the standard sprocket 500 at the same abutment position. Therefore, the timing of engagement of the rollers 152 with the tooth gap bottoms of the sprocket is uniform, and the frequency of the vibration and noise generated corresponds to the number of sprocket teeth.
In the low noise chain transmission disclosed in Japanese Examined Patent Application No. Hei 7-18478, the angle, formed by a line tangent to the position at which the roller abuts a sprocket tooth surface when the roller seats on the tooth gap bottom and a line connecting the center of the roller and the center of said sprocket, is a small angle. Elastic deformation of the roller and/or the tooth surfaces takes place, and the impact is alleviated so that engagement noise is reduced. However since the roller becomes sandwiched between opposed tooth surfaces, smooth disengagement of the roller from the sprocket is prevented, and vibration of the chain takes place on the side at which the chain disengages from the sprocket, generating noise.
Objects of the invention are to solve the above-described problems, to provide a sprocket in which the vibration and noise are reduced and in which disengagement of a chain from the sprocket is smooth, and at the same time, to simplify manufacture of the sprocket, and reduce frictional noise.